1,550 research outputs found
Threshold voltage and space charge in organic transistors
We investigate rubrene single-crystal field-effect transistors, whose
stability and reproducibility are sufficient to measure systematically the
shift in threshold voltage as a function of channel length and source-drain
voltage. The shift is due to space-charge transferred from the contacts, and
can be modeled quantitatively without free fitting parameters, using Poisson's
equation, and by assuming that the density of states in rubrene is that of a
conventional inorganic semiconductor. Our results demonstrate the consistency,
at the quantitative level, of a variety of recent experiments on rubrene
crystals, and show how the use of FET measurements can enable the determination
of microscopic parameters (e.g., the effective mass of charge carriers).Comment: 4 pages, 4 figure
Fluctuation properties of laser light after interaction with an atomic system: comparison between two-level and multilevel atomic transitions
The complex internal atomic structure involved in radiative transitions has
an effect on the spectrum of fluctuations (noise) of the transmitted light. A
degenerate transition has different properties in this respect than a pure
two-level transition. We investigate these variations by studying a certain
transition between two degenerate atomic levels for different choices of the
polarization state of the driving laser. For circular polarization,
corresponding to the textbook two-level atom case, the optical spectrum shows
the characteristic Mollow triplet for strong laser drive, while the
corresponding noise spectrum exhibits squeezing in some frequency ranges. For a
linearly polarized drive, corresponding to the case of a multilevel system,
additional features appear in both optical and noise spectra. These differences
are more pronounced in the regime of a weakly driven transition: whereas the
two-level case essentially exhibits elastic scattering, the multilevel case has
extra noise terms related to spontaneous Raman transitions. We also discuss the
possibility to experimentally observe these predicted differences for the
commonly encountered case where the laser drive has excess noise in its phase
quadrature.Comment: New version. Accepted for publication in Physical Review
Dynamics of a stored Zeeman coherence grating in an external magnetic field
We investigate the evolution of a Zeeman coherence grating induced in a cold
atomic cesium sample in the presence of an external magnetic field. The
gratings are created in a three-beam light storage configuration using two
quasi-collinear writing laser pulses and reading with a counterpropagating
pulse after a variable time delay. The phase conjugated pulse arising from the
atomic sample is monitored. Collapses and revivals of the retrieved pulse are
observed for different polarizations of the laser beams and for different
directions of the applied magnetic field. While magnetic field inhomogeneities
are responsible for the decay of the coherent atomic response, a five-fold
increase in the coherence decay time, with respect to no applied magnetic
field, is obtained for an appropriate choice of the direction of the applied
magnetic field. A simplified theoretical model illustrates the role of the
magnetic field mean and its inhomogeneity on the collective atomic response.Comment: To appear in J. Phys.
Sub-Doppler resonances in the back-scattered light from random porous media infused with Rb vapor
We report on the observation of sub-Doppler resonances on the back-scattered
light from a random porous glass medium with rubidium vapor filling its
interstices. The sub-Doppler spectral lines are the consequence of saturated
absorption where the incident laser beam saturates the atomic medium and the
back-scattered light probes it. Some specificities of the observed spectra
reflect the transient atomic evolution under confinement inside the pores.
Simplicity, robustness and potential miniaturization are appealing features of
this system as a spectroscopic reference.Comment: 6 pages, 4 figure
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